They are very good quality. I bought some cheap $5AUD screen protectors from my local EBGames. They were hard to put on and had a huge air bubble around the edge, and after about an hour of Brain Training by my mother and my sister, the bottom screen protector was scratched so badly it was hard to see through it. I'm fairly sure it damaged the stylus as well.
The HORI protectors, however, suck onto the screen perfectly without bubbles as long as you clean dust off it first. I've had my set for about 3 weeks, and the touch screen protector's only got a few light scratches on it. I'm fairly sure those are from my worn down stylus, so I'm buying a new stylus set when I get a chance.
They are also pretty cheap. $10AUD from www.play-asia.com including shipping to Australia.
One thing I noticed about the invisibleShield material is that it's kind of rubbery, so it gives some resistance to the stylus. Not so bad that the stylus is hard to move, but it definitely didn't glide as easily. I use the full DS invisibleShield cover, but didn't use the ones for the screen and used the Hori ones instead.
That's what I really like about the Hori protectors. Almost from the minute you put them on you forget they're even there. It's pretty hard to get any bubbles with them, and the surface is just as smooth as the actual DS screen, maybe even smoother.
I had some left overs from the package i bought for my cell phone hehe so i am using them right now. It has scratches like no other. I might have to go pick up the 5 pack from hori and my target. Not sure as to when b/c honestly when the game is on i dont notice the scratches.